A Technical Evaluation of Fingerprint Scanners

Several fingerprint scanners have been recently introduced in the biometric market, so that it is quite difficult for inexpert users to understand the technological differences and the features which determine the scanner quality and performance.

The most widely used scanner technologies are: optical and capacitive

Capacitive scanners are characterized by a small sensing area, not robust with respect to mechanical damage and electrostatic charges, requiring very high maintenance costs if the sensing element wears out. They need more frequent cleaning, to remove from the sensor surface deposits of grease or dirt left from the fingers, which highly deteriorate the quality of the acquired images. They are more suitable for use with cell phones and personal digital assistants.

Optical scanners use USB or parallel port connections and can stand alone or be built into other peripherals. The optics-based system is physically stronger than semi-conductor-based systems in terms of impact-resistance, scratch-resistance, weather-durability and corrosion-resistance. Physical strength is a KEY factor for versatile outdoor usage. They perform better and are more durable than silicon-chip devices.

Over long periods of time, the optical-based scanners are more durable, require less maintenance, are more accurate with their failure rates, i.e., FAR (False acceptance rate) and FRR (False rejection rate) at the bare minimum.

FX2000 is an optical scanner. It’s features are given below, detailing why it is most superior in quality as well as in performance:

1. RESOLUTION : The technical parameter to which people generally attribute the main importance is the resolution, that is the number of pixel per inch (dpi) characterizing the acquired images. Intuitively, the resolution indicates the magnification or zoom factor of the scanner. A 500 dpi resolution is required by FBI-compliant systems. 250-300 dpi is probably the minimum resolution which allows minutiae to be detected in the fingerprint pattern.

The figure below compares the same fingerprint portion as acquired at 500 dpi and at about 350 dpi. FX 2000 is a scanner that operates at a resolution of 569 dpi, even greater than that required by FBI.

2. SENSING AREA : A fundamental technical feature is undoubtedly the size of the scanner sensing area; in fact, this parameter determines the size of the fingerprint portion which can be acquired by the scanner. This parameter usually lies in the range 1.0x1.0 squared inches of some professional models to about 0.42"x0.42" of some low profile models. It is worth noting that in the second case the captured portion is about 5.6 smaller than in the first one. Why a wide sensing area is so important? The size of an average fingerprint is about 0.5"x0.7" (smaller for children and females, larger for adult males) and therefore the acquisition of a fingerprint with a sensing area smaller than 0.5"x0.7" produces a partial fingerprint; this effect gets worse by the problem of partial overlapping between different acquisitions of the same finger due to unavoidably positioning changes. The figure shows 4 different acquisitions of the same finger as produced by an optical scanner with a 0.51"x0.51" area

It is well evident from the above figure that the common part of the fingerprint is quite limited: as a consequence, the accuracy and reliability of fingerprint recognition are heavily compromised. The small overlapping between fingerprint acquired at different times is undoubtedly one of the main causes of false rejections (FRR) in fingerprint-based biometric systems.

Our FX 2000 scanner is characterized by a wide sensing area of 0.98"x0.52" which allows most of the fingerprints to be entirely acquired.

The following figure compares the same fingerprint as acquired by the FX2000 scanner with a typical capacitive scanner.

3. Besides the resolution and the size of the sensing area, the images produced by a fingerprint scanner are characterized by other factors which determine their neatness, contrast and geometric distortion. A deep analysis of these aspects is beyond the aim of this document.

4. Another important feature, especially in high security application, is the protection of the data and images which are exchanged between the scanner and the connected PC. The low computation power of the micro controllers (often 8 bits) mounted on most of the common models, makes it impossible to implement state-of-the-art encryption techniques. The FX 2000 scanner is equipped with a 32 bit RISC processor (20 MIPS) and implements very reliable symmetric encryption algorithms with 128-bit keys

Fingerprint recognition and Performance

5. Multi-modal matching: The most discriminant fingerprint features are the so called minutiae, and as several matching algorithms do, FX2000 performs a minutiae matching for determining the degree of similarity of two fingerprints. Unfortunately, sometimes minutiae cannot be reliably extracted due to the poor quality of the fingerprints (elderly people, manual workers, or other subjects whose ridge-line thickness is very low). The figure shows some fingerprints where minutiae extraction is critical and which also a human expert cannot easily process.

In such cases, a matching algorithm, which relies on minutiae only, is not able to decide with enough reliability whether two fingerprints belong to the same individual or not, and often produces a false rejection (thus increasing FRR).

FX2000 uses, besides minutiae, further 4 types of features which encode the ridge-line flow and density, and the ridge-line shape in some interesting regions, etc. The experimentation performed proved that these features can significantly help the "matcher", increasing its decision confidence especially when:

a. fingerprints are low quality

b. the fingerprint pattern is characterized by several scratches or damaged regions

c. the overlapping between two samples of the same finger (common area) is small and the number of common minutiae is small as well.

d. Direct gray-scale feature extraction: FX2000 extracts the minutiae and the other features used for the matching directly from the gray-scale image (without binarization and thinning). In the paper "Direct Gray-Scale Minutiae Extraction in Fingerprints", IEEE transaction on Pattern Analysis Machine Intelligence, 1997, it is shown that a direct extraction causes a smaller loss of information (and consequently gives a better accuracy) and, at the same time, decreases the computation time. FX2000 feature extraction algorithms represent an evolution and robust variant of those proposed in the above mentioned paper.

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